9:45 AM - 10:00 AM
[HCG24-04] Transition of groundwater flow field and residence time distribution during sea-level change
Keywords:sea-level change, seawater-freshwater interface, residence time, fossil seawater, numerical simulation of groundwater flow, geological disposal
Coastal areas are considered promising for HLW disposal site because wastes can be transported easily. Groundwater may migrate very slowly in coastal area because of low hydraulic gradient. However, migration of groundwater will be affected by sea-level changes (SLC) during long period. Numerical simulations have been conducted considering the effect of SLC to evaluate the migration of groundwater in coastal area. We have simulated not only concentrations of chloride ions in groundwater but also residence time of groundwater so that the retention of groundwater can be evaluated properly. This presentation describes the hydraulic conductivities to exist as groundwater which has lower mobility such as fossil seawater. The transition of the distributions of chloride ions and residence time of groundwater with SLC are discussed in relation to hydraulic conductivities.
A transient simulation of groundwater flow considering seawater density and residence time was performed using a 2D-cross section model. In the model, it is assumed that the medium (bedrock) is homogeneous with constant topographic gradient. The residence time of groundwater is simulated by generating a target material at constant rate in the medium. FEFLOW was used to simulate the groundwater and mass transport. It is assumed that the SLC takes one cycle for 120,000 years. Furthermore, the sea level was set +5 m (EL.) at maximum transgression and the sea level was set -120 m (EL.) at minimum regression. In this simulation, the SLC cycles ware calculated for three cycles. Boundary conditions on ground and sea surfaces were dynamically selected to correspond to changes of sea levels with elapsed time.
The position of seawater-freshwater interface was found to be controlled by the change of SLC when hydraulic conductivity was set 1.0×10-6 m/s. The residence time of groundwater originated from sea and fresh water were seemed to be close because they were mixed well by the change of SLC. Only the shallow part of groundwater originated from seawater was mixed with that from freshwater. In addition, residence time of groundwater originated from seawater got long when hydraulic conductivity was set 1.0×10-9 m/s. These results means that the effect of SLC on distributions of chloride ions and residence time of groundwater was strongly dependent on hydraulic conductivity of the bedrock. Thus, it can be inferred that the effect of SLC is not significant in deep part of geological layers if hydraulic conductivity is low. These results can explain why very old groundwater originated from seawater can be observed even under the effect of SLC during long period.
This study was carried out under a contract with the Ministry of Economy, Trade and Industry (METI) as part of its R&D supporting program titled " The project for validating assessment methodology of geological disposal system in coastal region (FY 2021) Grant Number JPJ007597.
A transient simulation of groundwater flow considering seawater density and residence time was performed using a 2D-cross section model. In the model, it is assumed that the medium (bedrock) is homogeneous with constant topographic gradient. The residence time of groundwater is simulated by generating a target material at constant rate in the medium. FEFLOW was used to simulate the groundwater and mass transport. It is assumed that the SLC takes one cycle for 120,000 years. Furthermore, the sea level was set +5 m (EL.) at maximum transgression and the sea level was set -120 m (EL.) at minimum regression. In this simulation, the SLC cycles ware calculated for three cycles. Boundary conditions on ground and sea surfaces were dynamically selected to correspond to changes of sea levels with elapsed time.
The position of seawater-freshwater interface was found to be controlled by the change of SLC when hydraulic conductivity was set 1.0×10-6 m/s. The residence time of groundwater originated from sea and fresh water were seemed to be close because they were mixed well by the change of SLC. Only the shallow part of groundwater originated from seawater was mixed with that from freshwater. In addition, residence time of groundwater originated from seawater got long when hydraulic conductivity was set 1.0×10-9 m/s. These results means that the effect of SLC on distributions of chloride ions and residence time of groundwater was strongly dependent on hydraulic conductivity of the bedrock. Thus, it can be inferred that the effect of SLC is not significant in deep part of geological layers if hydraulic conductivity is low. These results can explain why very old groundwater originated from seawater can be observed even under the effect of SLC during long period.
This study was carried out under a contract with the Ministry of Economy, Trade and Industry (METI) as part of its R&D supporting program titled " The project for validating assessment methodology of geological disposal system in coastal region (FY 2021) Grant Number JPJ007597.